Gibberellin A1 (GA1) levels drop significantly in wild-type pea (Pisum sativum) plants within 4 h of exposure to red, blue, or far-red light. This response is controlled by phytochrome A (phyA) (and not phyB) and a blue light receptor. GA8 levels are increased in response to 4 h of red light, whereas the levels of GA19, GA20, and GA29 do not vary substantially. Red light appears to control GA1 levels by down-regulating the expression of Mendel's LE (PsGA3ox1) gene that controls the conversion of GA20 to GA1, and by up-regulating PsGA2ox2, which codes for a GA 2-oxidase that converts GA1 to GA8. This occurs within 0.5 to 1 h of exposure to red light. Similar responses occur in blue light. The major GA 20-oxidase gene expressed in shoots, PsGA20ox1, does not show substantial light regulation, but does show up-regulation after 4 h of red light, probably as a result of feedback regulation. Expression of PsGA3ox1 shows a similar feedback response, whereasPsGA2ox2 shows a feed-forward response. These results add to our understanding of how light reduces shoot elongation during de-etiolation.
The levels of GA 1 , 3‐epiGA 1 and GA 8 in genotypes Le, le and le d of Pisum sativum L. were determined by gas chromatography‐selected ion monitoring (GC‐SIM) after feeds of [ 3 H, 13 C]‐GA 20 to each genotype. The levels of endogenous and [ 13 C]‐labelled metabolites were determined by reverse isotope dilution with unlabelled GA 1 , 3‐epiGA 1 and GA 8 . The results demonstrate a quantitative relationship between the level of GA 1 and the extent of elongation both on a per plant and a per g fresh weight basis. These results are consistent with previous findings in peas and other species possessing a predominant early 13‐hydroxylation pathway for GA biosynthesis. The levels of 3‐epiGA 1 also decreased in the genotypic sequence Le, le, le d although not as rapidly as for the level of GA 1 . This may suggest that the alleles at the le locus also influence the formation of 3‐epiGA 1 .
Ross, J. J. and Reid, J. B. 1989. Internode length in Pisum. Biochemical expression of the Le gene in darkness. The Le gene appears to be biochemically expressed in dark‐grown pea ( Pisum sativum L.) plants since the previously reported difference in metabolism of [ 3 H]‐GA 30 between light‐grown Le and Le plants was also observed in darkness. Furthermore, both light‐ and dark‐grown Le plants contained more endogenous GA 1 , ‐like substance than did comparable Le plants. Darkness did not appear to significantly increase the accumulation of GA 1 , in either Le or Le plants, although confirmation of GA 1 levels by gas chromatography‐selected ion monitoring is still required. The results support previous findings that the overall metabolism of [ 3 H]‐GA 20 , is accelerated by darkness. The evidence presented here supports previous suggestions that darkness acts on internode length by increasing some aspect of GA sensitivity.
Plant growth chambers are commonly used to minimize environmental variation but the light sources used vary considerably from natural light and from each other. Incandescent globes are often used to add more far‐red light, with the aim of producing a more natural red to far‐red ratio (R:FR), but also add to thermal load. High‐intensity discharge lamps are often used to produce higher irradiances, more akin to natural light, but the thermal implications are rarely considered because air temperature is controlled. This paper examines the spectral properties and thermal implications of growth chamber light sources and takes a whole‐plant physiology approach, by examining growth responses of a photoperiodic pea line ( Pisum sativum L. cv. Torsdag) in the same growth chamber type under different light sources – in essence using plants to study the controlled environments rather than vice‐versa. High R:FR delayed flowering and inhibited internode extension in pea. However, the addition of far‐red‐rich incandescent globes in the proportions provided in the growth chambers (400–500 W) did little to reduce R:FR, did not induce earlier flowering and actually further inhibited internode length. Leaflet size and yield were significantly reduced. While air temperature was maintained at 20°C in all experiments, radiant temperature was significantly higher under high irradiance and/or with incandescent added, and soil temperatures were elevated. Growth responses under these lights were similar to the effect caused by elevating the air temperature. An alternative method of controlling R:FR, without thermal load implications, using light‐emitting diodes is described.
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